1. Field of Invention
The present invention relates to the field of magnetic imaging. More specifically, it relates to the generating of high quality magnetic imaging using lower quality laboratory equipment that is limited to only measuring the absolute value of an observed magnetic field.
2. Description of Related Art
Electric current source estimation is a common problem in various areas of electromagnetic imaging technology. One area of particular interest is the imaging of electric fields emanating from living tissue. Living organisms generate electric impulses, which result in electric fields, and electric imaging (or electric field imaging) makes it possible to capture images of these electric fields (hereinafter termed electric images). Electric imaging has found wide application in the medical field, where it is often desirable to reconstruct an electric impulse, or dipole, from its electric image.
As it is known in the art, an electric current impulse generates a magnetic field. Thus, organisms that generate electric impulses also generate magnetic fields, and magnetic imaging (or magnetic field imaging) makes it possible to capture images of these magnetic fields (hereinafter termed magnetic images). The study of such magnetic fields in living organisms, or living tissue, is generally known as biomagnetism. Of particular interest in the field of biomagnetism is the magnetic imaging of the human brain and the human heart.
The development of electric imaging and magnetic imaging technology permits the detection and analysis of electrophysiological processes in the brain, heart and other nerve systems. Recording (i.e. imaging) of the electromagnetic fields from such tissues is typically accomplished by placing multiple electric (field) sensors or magnetic (field) sensors around, or over, the tissue being studied. For example, electroencephalography (EEG) uses electric sensors placed around the brain to record electric images of brain tissue, and electrocardiography (ECG or EKG) uses electric sensors placed over the chest to record electric images of heart tissue. Similarly, magnetoencephalography (MEG) uses magnetic sensors placed around the brain to record magnetic images of brain tissue, and magnetocardiography (MCG) uses magnetic sensors placed over the chest to record magnetic images of heart tissue. Examples of an MEG unit and an MCG unit are illustrated in FIGS. 1A and 1B, respectively.
With reference to FIG. 1A, an MEG system consists of a large number (usually 300 or less) of magnetic sensors arranged in a spherical shape (to be fitted around a human head) to provide a high spatial resolution for measurements. The MEG system measures magnetic fields created by brain nerve activity. Each magnetic sensor measures a one-dimensional (1D) magnetic waveform, Bz, in the radial direction.
With reference to FIG. 1B, an MCG system may include a small number (usually 64 or fewer) of magnetic sensors (each sensor is typically a Superconducting Quantum Interference Device, or SQUID) arranged as a sensor planar array. Each SQUID sensor measures a one-dimensional (1D) magnetic waveform (Bz) in the z direction, as illustrated by (x,y,z) axes. The MCG device is usually placed above and within 10 cm of a patient's chest in a location over the patient's heart. Electric current [i.e. electric impulse(s)] in the heart generates a magnetic field B that emanates out from the patient's torso. Each SQUID sensor measure the z-component (i.e. Bz) of the emanating magnetic field B that reaches it. That is, each SQUID sensor measures a 1D magnetic waveform in the z direction, including the positive or negative sign of the magnetic field at the sensor's location.
Compared to electric imaging (or recording) technology such as EEG and ECG, magnetic imaging technology such as MEG and MCG would be preferred due it being more non-invasive and providing a two-dimensional (2D) image (by virtual of the x-y plane of SQUID sensors) at a given point in time. Moreover, the magnetic field generated outside of the human body is not distorted in the direction perpendicular to the body surface (e.g. the radial direction in FIG. 1A and the z-direction in FIG. 1B), due to the magnetic property of body tissue. Thus magnetic imaging has the possibility of being more accurate and sensitive to weak electric activity within the body than is electric imaging.
Magnetic imaging, however, has several limitations. The magnetic fields generated in living tissue are very small. For example, a cerebral magnetic signal is a billion times smaller than the Earth's magnetic field. Consequently, magnetic fields generated in living tissue can easily be obscured by ambient magnetic noise. This can be addressed by using a magnetically-shielded room to reduce background noise. However, one still needs very sensitive magnetic sensors to detect the small magnetic fields emanating from human tissue.
As is explained above in reference to FIG. 1B, an array of SQUIDs are used in the construction of an MCG system. Unfortunately, the size and complexity of SQUIDs limit MCG systems to a relatively small number of SQUID sensors. This limits the resolution of a magnetic image, i.e. an MCG map.
As a result, an MCG system can typically produce only low resolution (low-res) 2D MCG maps. Typically, these low-res 2D MCG maps are not sufficient for diagnosis purposes. For example, a 64 channel Hitachi™ MCG system with a 25 mm sensor interval (as described in “Newly Developed Magnetocardiographic System for Diagnosing Heart Disease”, by Tsukada et al., in Hitachi Review, 50(1):13-17, 2001) only measures an 8×8 MCG map (i.e. it has an 8×8 array of 64 measurement points, or captures). One solution is to increase the number of sensors, but this is very difficult in practice due to the physical size of the sensors and system design constraints.
One approach to overcoming this physical limitation is to approximate a high-resolution (hereinafter, high-res) magnetic image from the low-res image created by the limited number of magnetic sensors. Thus, a necessary step in MCG is generating a high-res, 2D MCG image, or map, from a low-res, 2D MCG image, or map.
Two image examples, L and R, of high-res 2D MCG images generated from low-res images are shown in FIG. 2. Left image L shows the tangential image of a generated high-res MCG image of a healthy heart. The maximal point (i.e. strongest point) within image L indicates the 2D location (or source) of electric current in the heart. Thus, high-res MCG images permits doctors to directly “see” the electrical activity in the heart. Right image R shows the tangential image of a high-res MCG image of an unhealthy heart. It differs significantly from left image L of a healthy heart, and thus provides important cues for diagnosis. Compared to low-res MCG maps, high-res MCG images provide more diagnostic significance.
One way to generate a high-res magnetic field image from a low-res magnetic image is by interpolation. Most modern MCG systems use curve fitting interpolation methods between observed measurements of the electromagnetic sensors to construct high-res 2D MCG images from the low-res 2D MCG maps, such as described in “Magnetocardiographic Localization of Arrhythmia Substrates: A Methodology Study With Accessory Path-Way Ablation as Reference”, by B. A. S. et al., in Ann Noninvasive Electrocardiol, 10(2):152-160, 2005, and described in “Evaluation of an Infarction Vector by Magnetocardiogram: Detection of Electromotive Forces that Cannot be Deduced from an Electrocardiogram”, by Nomura et al, in Int. Congress Series, 1300:512-515, 2007. Unfortunately, the accuracy of curve fitting methods is typically limited.
As described above, magnetic imaging provides images of the magnetic field at a given time and depth within a tissue. Oftentimes, it would be beneficial to identify the electric current impulse (or current impulse) responsible for the observed magnetic field. Trying to identify the current impulse that generated an observed magnetic image is termed the inverse problem. That is, using the obtained magnetic field measurements at different sites, one attempts to estimate the location and moment of the current source that generated the observed (i.e. the measured) magnetic field.
There are a number of difficulties involved in addressing the inverse problem. According to the Helmholtz reciprocity principal, the inverse problem for MCG is an ill-posed problem unless the prior electric currents and their number are known. For example, a trivia case that assumes a single electric current located at the world origin and far from the sensor array is described in Magnetocardiographic Localization of Arrhythmia Substrates: a Methodology Study with Accessory Pathway Ablation as Reference, Europace, 11(2):169-177, 2009, R. J. et al. This situation cannot be satisfied in practice.
Addressing the inverse problem usually requires that it be simplified by making use of regularization methods (as described in “MEG Inverse Problem with Leadfields”, 15th Japan Biomagnetism Conference, 13(1):42-45, 2000, by A. Matani) and that the position of current sources be given a priory (as described in “An Optimal Constrained Linear Inverse Method for Magnetic Source Imaging”, Nuclear Science Symposium and Medical Imaging Conference, pages 1241-1245, 1993, by P. Hughett).
Irrespective of the technique used to address the inverse problem, all the techniques make use of captured magnetic images, which provide information of the magnetic flux at a particular point in time at a given depth within a tissue. The capturing of these magnetic images requires specialized, magnetically-shielded rooms and the use expensive MCG equipment comprised of SQUID sensors.
As is explained above, however, SQUID sensors are delicate and complicated devices, and thus not easy to procure for typical laboratory settings. This limits experimental research into the field of magnetic imaging for medical purposes. What is needed is a method of facilitating laboratory experimentation and research in the field of MCG imaging without the use of SQUID sensors.
An object of the present invention is to provide a method of making use of readily available laboratory magnetic sensors to create magnetic images that provide similar information as those provided by MCG images.
Another object of the present invention is to derived positive and negative sign information for a magnetic image from purely magnitude data.
A further object of the present invention is to provide a method of addressing the inverse problem using readily available laboratory magnetic sensors.
A further object of the present invention is to reduce the requirements on laboratory magnetic sensors, by permitting the use of simplified magnetic sensors that only measure the magnitude of a magnetic flux and provide no information regarding its positive or negative sign, i.e. its polarity or direction.